Well the sponsors could be Russians living abroad, couldn't they?
But on principle YES, imo Navalny is himself corrupt and lacks principle if he accepts money from foreign groups that do not have the best interests of the people he is supposed to represent, at heart. In this, Muscovites.
The only way to have democracy is to LIMIT the funds that are available to ALL candidates involved, so that none of them have more money at their disposal than the others. That way they are forced to rely on their message and programme, not on flashy ads or cheesy campaigns.
Nobody should be able to use their own private funds if they happen to be rich, or accept donations from others, corporations, foreign organisations etc.
So part of the election must be ongoing audits of what funds the parties and candidates spend.
Otherwise the interests of whatever groups have the most money will be able to turn around public opinions and win.